Steric size in conformational analysis. Steric compression analyzed by circular dichroism spectroscopy

The relative steric size of methyl, ethyl, isopropyl, tert-butyl, phenyl, a nd benzyl groups has been determined from a sensitive tetrapyrrole model an d exciton coupling circular dichroism (CD) measurements. Unlike the classic al cyclohexane model, from which the relative steric demand of functional g roups has been assessed quantitatively (A-values) and is based on the prefe rence for equatorial vs axial orientations, the bilirubin model assesses su bstituent size from head-to-head steric compression. Thus, exciton CD ampli tudes of a set of sensitive anti-chiral alpha (R/S)-substituted-beta'(S)-me thylmesobilirubins-XIII alpha (1-6) suggests an apparent relative steric si ze: tert-butyl similar to isopropyl > phenyl similar to ethyl > benzyl > me thyl. The order is somewhat different from that obtained by measuring equat orial vs axial configuration preferences on substituted cyclohexanes by NMR spectroscopy: tert-butyl much greater than phenyl > isopropyl > ethyl simi lar to benzyl similar to methyl.